My dersa clam has completely vanished! Saw it two days ago in the same spot it's been for two months all happy and fully open and now can't find it anywhere in my tank! Not even the shell! Are they that mobile I've got a lot of rock in the tank so I'm not able to see in the back but can't figure out how it would get back there either. Hate to see it get stuck somewhere with little light and die! At a loss!